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Synlett 2008(14): 2169-2171  
DOI: 10.1055/s-2008-1078246
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Palladium-Mediated Synthesis of Phenanthridines: The First Report of Palladium Insertion into Imidoyl Selenides

W. Russell Bowman, Jessica E. Lyon, Gareth J. Pritchard*
Department of Chemistry, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK
Fax: +44(1509)223925; e-Mail: G.J.Pritchard@lboro.ac.uk;
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Publication History

Received 2 June 2008
Publication Date:
31 July 2008 (online)

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Abstract

Imidoyl phenyl selenides have been shown to be precursors to phenanthridines, when treated with Pd(PPh3)4 under reflux in toluene in the presence of a base, giving reasonable yields. It is proposed that insertion of palladium into the C-Se bond is followed by cyclisation onto a phenyl ring and subsequent aromatisation by elimination of HPdSePh.

Key words

heterocycles - palladium - phenanthridines - imidoyl sel­enides - cyclisations

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17

Typical Procedure for the Synthesis of Phenanthridines To a mixture of Pd(PPh3)4 (83 mg, 0.072 mmol) and Et3N (0.13 ml, 0.90 mmol) was added the imidoyl selenide 2e (80 mg, 0.18 mmol) in toluene (2 mL). The mixture was refluxed for 48 h, and after cooling to r.t., H2O (10 mL) was added. The reaction mixture was extracted with CH2Cl2 (2 × 10 mL). The combined organic phases were washed with a sat. aq solution of NaHCO3 (10 mL), brine (10 mL), dried (MgSO4), filtered, and the solvent removed under reduced pressure. The crude product was further purified by column chromatography (neutral alumina, 20% CH2Cl2 in hexane) to yield the desired 6-(4-chlorophenyl)phenanthridine (1e, 24 mg, 46%) as colourless crystals; mp 152-154 ˚C (Lit.¹¹ mp 160-161 ˚C). IR (KBr): 2924, 1608, 1485, 1361, 1091, 829, 752, 721 cm-¹. ¹H NMR (400 MHz, CDCl3): δ = 7.54 (2 H, m, ArH), 7.64 (1 H, m, ArH), 7.70 (3 H, m, ArH), 7.77 (1 H, m, ArH), 7.88 (1 H, m, ArH), 8.06 (1 H, dd, J = 0.8, 8.4 Hz, ArH), 8.23 (1 H, dd, J = 1.2, 8.4 Hz, ArH), 8.63 (1 H, dd, J = 1.6, 8.4 Hz, ArH), 8.72 (1 H, d, J = 8.4 Hz, ArH). ¹³C NMR (100 MHz, CDCl3): δ = 122.0 (ArCH), 122.4 (ArCH), 123.8 (ArC), 125.0 (ArC), 127.2 (ArCH), 127.3 (ArCH), 128.5 (ArCH), 128.7 (2 × ArCH), 129.0 (ArCH), 130.4 (ArCH),130.7 (ArCH), 131.1 (2 × ArCH), 133.5 (2 × ArC), 134.9 (ArC), 143.8 (ArCCl), 160.0 (ArC=N). LRMS-FAB: m/z (%) = 290 (67) [M + H+], 289 (38), 176 (22), 155 (24), 154 (100), 138 (25), 137 (48), 136 (61). HMRS: m/z calcd for C19H13NCl: 290.0736; found: 290.0732.